Process of producing high-nitrogen alloy steel



Sept. 24, 1968 J. FREHSER ET AL PROCESS OF PRODUCING HIGH-NITROGEN ALLOY STEEL Filed April 27, 1965 STEEL MELT VAcuuM MOLD HE LT CHAMBER INVENTOR Jan? Frchse? 0! ed ATTORNEY United States Patent 3,402,756 PROCESS OF PRODUCING HIGH-NITROGEN ALLOY STEEL Josef Frehser, Albert Bohlergasse 8, and Christian Kubisch, Steinerhof-Allee 7, both of Kapfenberg,

Styria, Austria Filed Apr. 27, 1965, Ser. No. 451,324 Claims priority, application Austria, May 12, 1964, A 4,12/64 5 Claims. (Cl. 164-55) ABSTRACT OF THE DISCLOSURE Described herein is a process of producing high-nitrogen alloy steel by preparing a steel melt sufiiciently overheated to cause it to remain molten during the succeeding steps of process. The melt is poured into a mold where it can be vacuum degassed, then nitrogen is blown into the melt to create superatmospheric pressure, and the melt is solidified in the nitrogen environment under the superatmospheric pressure.

Nitrogen alloy steels have the advantage that the alloying element is relatively inexpensive and improves mechanical properties. The difiiculties involved in the introduction of nitrogen into the steel may be eliminated by various means. The oldest and best known method of introducing nitrogen is to add high-nitrogen alloys to the molten steel. This method has the disadvantage of resulting in more or less high-alloy steels. The second possibility resides in melting under pressure. In this process the steel is molten under pressure in a nitrogen atmosphere so that it is saturated with nitrogen in dependence on the gas pressure of nitrogen and on temperature. This process has the disadvantage that part of the nitrogen is liberated when the steel is being poured. The casting operation involves a considerable loss of nitrogen and results in a porous ingot. In order to eliminate this disadvantage, it has been suggested to conduct the melting and pouring operations in a pressure vessel. This pressure vessel, however, had to be so very large and the two operations performed under pressure in said pressure vessel had to be fully mechanized so that the structural expenditure was disproportionately high compared to the benefit achieved by the incorporation of nitrogen.

Tests have shown that it is not necessary to resort to the expensive melting under pressure when it is possible to cool the steel under nitrogen pressure. The high melt temperature results in a fast reaction between the steel and its gaseous environment so that the desired increase in nitrogen content is rapidly effected when this step is carried out under pressure. Before the introduction of nitrogen under pressure, it is desirable to outgas the steel in the same vessel by a vacuum treatment so that the nitrogen subsequently flowing in under pressure is taken up even more rapidly and more intensively, oxygen having an adverse effect on the introduction of nitrogen.

Thus, the present invention relates to a process of producing nitrogen alloy steels in a pressure vessel, which has the strength and pressure resistance required for the pressure to be maintained therein and consists of a pressure-resisting steel body, in which an ingot mold having a refractory lining is placed, into which the steel, in which nitrogen is to be incorporated, is poured in a slightly overheated state. The vessel is then pressure-tightly closed 3,402,756 Patented Sept. 24, 1968 ice and outgassed by a vacuum. After outgassing, nitrogen is blown in by a lance or a tuyere bottom or a porous bottom block of the ingot mold itself at a pressure which depends on the amount of nitrogen to be incorporated. In this ingot mold, the steel, which has been pressurized or charged with pressure gas, must solidify under the desired, set pressure. In this way the theoretically calculated nitrogen values can be approximately attained.

Prior art literature suggests that the steel industry has long recognized nitrogen contents of 3%. In this regard, reference is made to the German publication entitled Berg-und Huttenmaennische Monatschfte, November 1963, pages 375376.

The drawing schematically shows the process of producing high-nitrogen alloy steels.

The nitrogen pressures which are required are determined by Sieverts square root law Percent nitrogen: 1N

Pure iron would be saturated with nitrogen in an amount of 0.215% under a pressure of 25 kg./sq. cm. superatmospheric pressure and at 1600 C. The solubility of nitrogen is highly increased, however, by the elements manganese, silicon, chromium and vanadium. It is possible without difficulty by means of this pressure casting process to introduce nitrogen to a content of 0.8% into a steel which contains, e.g., 0.3% carbon, 0.3% silicon, 18% manganese, 15% chromium and 2% nickel at the relatively low pressure of 25 kg./ sq. cm.

The superatmospheric nitrogen pressure may be released through a plant for regenerating nitrogen. After this release, the pressure vessel is opened, the ingot mold is lifted by a crane and the ingot is stripped in known manner. The stripped ingot may be subjected to any further processing in accordance with its intended use.

This process of incorporating nitrogen may also be carried out without preceding outgassing by a vacuum treatment.

What is claimed is:

1. A process of producing high-nitrogen alloy steel, which comprises the steps of preparing a steel melt, pouring said melt in a slightly overheated state into an ingot mold contained in a chamber, then introducing nitrogen into said mold through said melt, obtaining superatmospheric pressure in the chamber by said introduced nitrogen so as to saturate said melt with the desired percentage of nitrogen, and then solidifying said melt in said mold under said pressure.

2. A process as set forth in claim 1, which comprises vacuum-outgassing the melt in said mold before said nitrogen is introduced into said chamber.

3. A process as set forth in claim 1, in which said prepared steel melt contains approximately 0.3% carbon, 0.3% silicon, 18% manganese, 15% chromium and 2% nickel.

4. A process as set forth in claim 3, in which a nitrogen pressure of 25 kg./sq. cm. superatmospheric pressure is obtained in said chamber and nitrogen is alloyed with said melt to a value of 0.8%.

5. A process as set forth in claim 1, in which said ingot mold is removed from said chamber when said melt has solidified therein and the resulting ingot is stripped from said mold.

(References on following page) References Cited UNITED STATES PATENTS Coffin 16466 Kinzcl 16466 Jackson et a1. 16457 X Drummond 75-122 Leroy et a1 26638 Pagonis 26638 4 2,021,979 11/1935 Arness 75130.5 2,696,433 12/1954 Tanczyn 75-1305 FOREIGN PATENTS 5 957,046 5/ 1944 France. 1,188,658 3/ 1959 France.

I. SPENCER OVERHOLSER, Primary Examiner.

V. K. RISING, Assistant Examiner. 

